1. Field of the Invention
The present invention relates to a tire air-pressure alarming device for alarming a tire air-pressure drop based on a result obtained by indirectly detecting the state of tire air pressure in a motor vehicle. It further relates to a tire monitored by the device and a control method for property values of the tire.
2. Discussion of the Related Art
Heretofore, as tire air-pressure alarming devices of this kind, there has been known one described in Japanese unexamined, published patent application No. 07-156621 for example. The known device comprises wheel speed detection means for detecting the wheel speeds of wheels of a motor vehicle, decision value deriving means for deriving a decision value based on the wheel speeds detected by the wheel speed detection means, air-pressure drop judging means for judging the tire air-pressure drop of the wheels based on the decision value derived by the decision value deriving means, and alarming means for raising an alarm when the tire air pressure of the wheels is judged by the air-pressure drop judging means to have dropped.
In the device, the air-pressure drop can be detected correctly by accurately and speedy compensating for initial differences of the tires which are caused by a dispersion of the tire property in production (including replacement of tires, air replenishment into the tires and the like). More specifically, as shown in FIGS. 1 and 2 of the aforementioned Japanese application, the speed Vi of each tire is detected (step S4), forward-rearward acceleration Ai is calculated based on the speed Vi when the same is larger than a threshold value VTH (step S7), and further, when the calculated forward-rearward acceleration is smaller than the threshold value VTH, it is judged whether or not the vehicle is in a straight-ahead traveling (step S9). As a result, when the vehicle is in the straight-ahead traveling, a rotational angular velocity Fi is stored in a rotational angular velocity area of a RAM for use in an initial compensation processing (step S10). The foregoing processing steps are repetitively executed until the traveling distance of the vehicle reaches a predetermined distance. When the traveling distance of the vehicle reaches the predetermined distance, a compensation coefficient Ci is calculated by the use of only the rotational angular velocities Fi being stored in the rotational angular velocity area (step S14). In accordance with the foregoing processing, the initial compensation processing can be executed more accurately since in the initial compensation, the rotational angular velocities Fi suitable for the initial compensation are selected and since the compensation coefficient Ci is calculated by using the selected rotational angular velocities Fi only.
In the compensation for the initial differences in the foregoing device, the dispersion of the tire property can be relieved by performing the compensation which takes traveling conditions into consideration, during the traveling of the vehicle. However, there arises a problem in that where the dispersion is large, it cannot be compensated to a full extent. Further, even if the initial differences were relieved, it is not carried out in the known device to relieve the dispersions (hereafter referred to as “property variation differences”) which are caused by vehicle speed, wheel load (load on one wheel) and property variation in tire air-pressure. This gives rise to another problem that variations in the vehicle speed, the wheel load and the tire air-pressure property make it unable to detect the tire air-pressure drop precisely. That is, since each tire is not controlled in properties thereof, decision values a disperse large with the tires set under a normal pressure, and the sensitivity to the decision values a also fluctuates large with the decrease in the tire pressure. Thus, further problems arise in that the air-pressure drop is detected in spite of the fact that the tires are sufficient in air pressure or that the air-pressure drop is not detected in spite of the fact that the tire air pressure is below a lower limit for judgment.